It's well known that sulfur is one of the main impurities of hydrocarbon raw materials. Its concentration in fuels ranges from hundredths of percent up to 10% or above [1.  H. K.       .—M.: Hayκa, 1984 (In English: Lyapyna N. K. Chemistry and physical chemistry of oil distillates of sulfur-organic compounds.—Moscow: Publishing House ‘Nauka’, 1984), 2.  Γ.Φ.   .—: Hayκa, 1986 (In English: Bolshakov G. F. Sulfur-organic compounds of oil.—Novosibirsk: Publishing House ‘Nauka’, 1986)].
It's generally known too that, during exhaustion of the world oilfields, percentage of sulfur compounds in crude oil increases. Petroleum products derived from crude oil contain, as a rule, appreciable concentration of sulfur. Combustion of such products yearly causes oxidation of about 40*106 tons of sulfur equivalent to approximately 80*106 tons of sulfur dioxide or 120*106 tons of sulfuric acid [ X.   ,    .   .—2000, T. 6, No 7, c. 42-46 (In English: Harlampydy Kh.E. Sulfur-organic compounds of oil, methods of cleaning and modification.—Soros educational magazine.—2000, v. 6, No 7, pp. 42-46].
Oxides of sulfur released to the environment from fuel combustion cause acid rains, which negatively affects the ecosystem of the Earth. Moreover, sulfur-containing compounds cause poisoning of catalysts used for after-burning of exhausts. As a result, large amount of nitrogen oxides and products of incomplete oxidation of hydrocarbons are released into the atmosphere.
Developed countries, which are most affected by these emissions, have determined that sulfur concentration in hydrocarbon fuels must be no more than 30 ppm in gasoline and no more than 15 ppm in diesel fuel. Still more requirements are applied to sulfur concentration in petroleum products, which are meant for fuel cells (in particular less than 10 ppm for solid oxide fuel cells, and less than 1 ppm for proton-exchange membrane fuel cells).
Preventing the emission of sulfur-containing compounds into atmosphere from fuel combustion is a major engineering and environmental problem, for which the solution of deep desulphurization of fuels is necessary.
Main sulfur-containing impurities of oils and petroleum products are mercaptans (RSH), sulfides (RSR′), disulfides (RSSR′), and cyclic sulfides (CnH2nS). More than 250 sulfur-containing compounds have been identified and many of them have been isolated from oils.
Many different methods for desulphurization have been proposed. For instance, oil refineries extensively use hydrodesulphurization (i.e. HDS-process) based on selective hydrogenolysis of C—S bonds using a catalyst such as Co—Mo/Al2O3 or Ni—Mo/Al2O3 at high temperature 320-380° C. and under a pressure of 3-7 megapascal. However, possibilities to improve the HDS-process by increasing catalyst's activity, optimization of operating practices and enhancement of equipment are now almost exhausted. In fact, latest reports about more efficient catalysts for the HDS-process were published at the beginning of XXI century [Kemsley, J. Targeting sulfur in fuels for 2006].
Unfortunately, any embodiment of the HDS-process generates hydrogen sulfide, which should be prevented. Further, said process is not able to remove effectively some sulfur-containing compounds (including cyclic and polycyclic monoalkylated and polyalkylated sulfur-containing compounds, such as alkyl benz- and alkyl dibenzthiophens which are usually present in kerosene, diesel fuel and vacuum gas-oil) though hydrogenolysis rate increases in series “mercaptans>disulfides>sulfides thiophens”. Moreover, the HDS-process is accompanied by hydrocracking and hydrogenolysis of olefins, dehydrogenation of naphthenic hydrocarbons and cyclodehydrogenation of alkanes that alters hydrocarbon composition of end products and causes degradation of gasoline's octane number or diesel fuels' cetane number. Increase of temperature and pressure in the HDS-process meant for deep desulphurization causes intensification of said side reactions.
Other desulphurization methods, such as biodesulphurization, extraction of sulfur with mineral and organic acids, desulphurization with ionic liquids, adsorption, etc were developed recently [see, for example: 1. Babich I. V.; Moulijn J. A. Fuel. 2003, 82 (6), 607-631 2. Song, C. Catal. Today. 2003, 86 (1-4), 211-263; 3.  . A.;  A.B. . 2004, 44 (2), 83-88 (In English: Aslanov L. A., Anisimov A. V. Neftekhimiya. 2004, 44 (2), 83-88)].
These methods are efficient only for removing of mercaptans, thioesters and disulfides but are practically unsuitable for removing of thiophens (especially benztiophens, dibenzthiophens and other thiophens, which include condensed cycles, or their substituted derivates).
Therefore, it is necessary to develop highly effective and inexpensive desulphurization methods, which do not practically alter composition and combustion efficiency of hydrocarbon fuels.
In particular, a special group of desulphurization methods based on oxidation of sulfur-containing compounds, adsorption of their oxidation products and separation of spent adsorbent are known [1.  A.X.,  B.P.      . 2005, 4, 42-43; (In English: Sharipov A.Kh, Nygmatullyn V. R. Chemistry and technology of fuels and lubricants. 2005, 4, 42-43); 2. Shiraishi Y., Yamada A., Hirai T. Energy and Fuels. 2004, 18 (5), 1400-1404; 3. Ke Tang et al. Fuel Proc. Technol. 2008, 89 (1) 1-6 3; 4. Ishihara A. et al. Appl. Catal. A: General. 2005, 279 (1-2), 279-2871 5. EP 1715025, 2006; 6. Velu S. et al. Energy and Fuels. 2005, 19 (3), 1116-1125; 7. Ma, C.; Zhou, A.; Song, C. Catal. Today. 2007, 123 (1-4), 276-284; 8. Liu B. S. et al. Energy and Fuels. 2007, 21 (1), 250-255, etc.].
A technical solution, which is closest to the proposed below invention, was described in US Patent Application No 2008/0257785 (Oct. 23, 2008; Varma R. S, Yuhong Ju, Sikdar S.). The known method for desulphurization of hydrocarbon fuels provides:
preparation of mixture of a powdered adsorbent based on at least one silicate and an oxidant that is a metal nitrate having high affinity to sulfur,
contact of this mixture with hydrocarbon fuel, which must be desulphurized, at temperature in the range from 20° C. to 50° C. under atmospheric pressure over the time that is sufficient for effective oxidation and adsorption of sulfur-containing compounds, and then
separation of spent adsorbent together with adsorbed oxidized sulfur-containing compounds from refined fuel.
Silicate can be selected from the group consisting of clay minerals such as montmorillonite, laumontite, bentonite, mica, vermiculite and kaolin, but usually modified montmorillonite K-10 from Aldrich Chemical Co. (USA) is used. Oxidant (in an amount from 5% to 35% of the adsorbent powder mass) can be selected from the group consisting of metals' nitrates such as iron (II) or (III), zinc (II), cadmium (II) and mercury (II), but mainly the mixture of iron nitrate (III) nonahydrate is used. Said mixture is prepared by careful grinding and mixing of selected solid oxidant and selected clay mineral practically ex tempora because activity of makeup mixture quickly decreases.
An experimental embodiment of aforesaid method showed that it is sufficiently effective for the purpose of hydrocarbon fuels purification from sulfur-containing compounds such as 2-methyl benzthiophen and 4,6-methyl dybenzthiophen even if their concentrations in treated fuel are low.
Unfortunately, use of said solid oxidants and necessity of their careful grinding with clay minerals practically before stirring of obtained mixtures and processed fuels complicates desulphurization substantially and increases the risk of environmental damage that can be caused by spent adsorbents (especially when they contain cadmium or mercury).